1. Field of the Invention
The present invention relates, generally to bi-directional overrunning clutch assemblies and, more specifically, to an actively controlled, bi-directional, overrunning clutch assembly having four modes of possible operation for use in motor vehicle driveline components such as transmissions, transfer cases, differentials and the like.
2. Description of the Related Art
Generally speaking, land vehicles require three basic components. These components include a power plant (such as an internal combustion engine), a power train and wheels. The power train""s main component is typically referred to as the xe2x80x9ctransmission.xe2x80x9d Engine torque and speed are converted in the transmission in accordance with the tractive-power demand of the vehicle. Transmissions include one or more gear sets which may include an inner gear, intermediate planet or pinion gears that are supported by their carriers, and outer ring gears. Various components of the gear sets are held or powered to change the gear ratios in the transmission. In addition to such planetary gear sets, driveline components may further include multi-disc friction devices that are employed as clutches or brakes. The multi-disc pack clutch is a friction device that is commonly employed as a holding mechanism in a transmission, transfer case or differential or the like. In addition, multi-disc friction devices also find use in industrial applications, such as wet brakes, for example, to brake the wheels on earth-moving equipment.
The multi-disc pack clutch or brake assembly has a clutch sub-assembly including a set of plates and a set of friction discs that are interleaved between one another. The plates and friction discs are bathed in a continual flow of lubricant and in xe2x80x9copen packxe2x80x9d mode normally turn past one another without contact. The clutch or brake assembly also typically includes a piston. When a component of a gear set is to be held, as for example during a particular gear range, a piston is actuated so as to cause the plates and friction discs to come in contact with respect to one another. In certain applications, it is known to employ several multi-disc pack clutch devices in combination to establish different drive connections throughout the transmission, transfer case, or differential to provide various gear ratios in operation, or to brake a component.
When the discs are not engaged, there often remains a differential rotational speed between the drive and driven members that the clutch or brake bridges. Relative rotation between the friction discs and the plates during open-pack mode creates drag. This condition results in parasitic energy losses, reduces the efficiency of the transmission, transfer case or differential, and ultimately results in lower fuel efficiency.
In addition to multiple friction devices, one-way clutches are frequently employed in transmissions, transfer cases, and differentials to selectively transmit torque in one rotational direction, but not in the opposite rotational direction. To this end, one-way clutches typically include an inner race, an outer race, and an engagement mechanism disposed therebetween. The engagement mechanism is operable to lock the inner and outer races together thereby transmitting torque in one relative direction. The engagement mechanism is further operable to allow freewheeling rotation between the inner and outer races in the opposite rotational direction. Engagement mechanisms commonly used in one-way clutches of the related art include pawls, sprags, and rollers. A cage, along with biasing members, such as springs, are also sometimes employed to retain the pawls, sprags, or rollers between the inner and outer races as well as to selectively assist in the change of operational modes between torque translation and freewheeling actuation of the clutch, depending on the direction of rotation between the inner and outer races.
As noted above, one-way clutches of this type have been employed in numerous applications in transmission, transfer cases, and differentials. For example, one-way clutches have been employed in conjunction with multiple friction clutches and planetary gear sets to effect low and reverse gear ratios in conventional transmissions. While this arrangement has worked well for its intended purpose, some disadvantages remain. For example, the friction clutch remains a source of significant parasitic losses due to inherent drag between the friction plates when the clutch is operating in xe2x80x9copen packxe2x80x9d mode. Still, the clutch is necessary for providing the proper holding torque in low and reverse gears. Accordingly, there remains a need in the art for a mechanism that can provide the appropriate holding torque for both low and reverse gears in the transmission and yet results in less parasitic losses which are presently attributable to the multiple plate friction clutch used for this purpose. In addition, there is a need in the art for a device that continues to perform the functions of the one-way clutch as described above, particularly where the output speed of the transmission exceeds the input speed resulting in engine compression braking.
One-way clutches have also been employed in transfer cases that provide full time, part time, and xe2x80x9con demandxe2x80x9d four wheel drive (4WD) capabilities. In these situations, the one-way clutch is typically disposed between the primary driveline and the secondary driveline. When the primary drive line attempts to over speed the secondary drive line, as will occur, for example, where the rear wheel is supported on a slick surface, such as ice and is spinning and the front wheels are solidly supported, the one-way clutch engages and transfers torque to the slipping wheel. In this way, 4WD is achieved, but in this case, only under circumstances that require it.
The use of a one-way overrunning clutch to selectively provide drive torque to a secondary driveline upon primary wheel slip has not, however, become a popular solution to part time 4WD vehicle requirements because of one problem: the clutch remains disengaged or inactive when reverse gear is selected unless, of course, the secondary driveline attempts to over speed the primary driveline. Thus, in a situation frequently requiring 4WD, that is, when the vehicle may need to be rocked or simply backed over terrain, a 4WD configuration utilizing a one-way overrunning clutch will simply not provide 4WD operation. This is a significant drawback of this clutch configuration.
Partially in response to this problem, bi-directional overrunning clutches have been proposed in the related art for use in these circumstances. These bi-directional overrunning clutch assemblies typically employ an inner race, an outer race, and a plurality of rollers as the engagement mechanism disposed therebetween. An actuator is employed to bias the rollers in one direction or another to allow freewheeling or torque translation in either rotational direction depending upon the position of the rollers. However, and while they appear to present a solution to certain problems identified above, bi-directional overrunning clutches have not been widely employed in transmission, transfer cases, and differentials of the related art. They are relatively mechanically complex and therefore generally more expensive than conventional one-way clutches. In addition, many of the designs considered in the related art simply do not operate at an acceptable level that meets the standards of today""s sophisticated driving public.
Accordingly, there remains a need in the art for a bi-directional overrunning clutch assembly that can provide torque translation in either rotational direction as well as one that may be employed as a substitute for conventional multi-disc friction devices presently known in the related art.
The disadvantages of the related art are overcome in a bi-directional overrunning clutch assembly of the present invention. The bi-directional clutch assembly includes an inner race, an outer race and an engagement mechanism that is supported therebetween. The engagement mechanism includes a first set of pawls and a second set of pawls operatively supported between said inner and outer races. In addition, the clutch assembly includes at least one actuating cam disposed adjacent the inner and outer races and including a plurality of disengagement portions as well as a plurality of engagement portions. The engagement and disengagement portions cooperate with the first and second pawls to (1) selectively disengage the first and second pawls to provide freewheeling relative rotation between the inner and outer races; (2) to selectively actuate at least one pawl of the first set of pawls so that torque is translated in a first rotational direction but allowing freewheeling relative rotation between the inner and outer races in a second rotational direction that is opposite to the first rotational direction; (3) to selectively actuate at least one pawl of the second set of pawls so that torque is translated in the second rotational direction opposite to the first rotational direction but allowing freewheeling relative rotation in the first rotational direction; and (4) to selectively actuate at least one pawl of the first and second sets of pawls so that torque is translated between the inner and outer races in both of the first and second rotational directions.
The bi-directional over-running clutch assembly of the present invention is particularly adapted for use in an automotive transmission having at least one shaft and at least one gear set that is operatively coupled to the shaft to provide low and reverse gear ratios. The transmission assembly includes a transmission casing for supporting the shaft and the gear set of the transmission assembly. The gear set includes a sun gear operatively coupled to a source of torque in the transmission assembly, a ring gear mounted for rotation about the sun gear and a plurality of pinion gears. The pinion gears are supported by a carrier for rotation about the sun gear and between the ring gear and the sun gear. The carrier is operatively coupled to the shaft. The bi-directional clutch assembly has an inner race that is operatively coupled to the ring gear of the gear set. The outer race is operatively coupled to the transmission casing and an engagement mechanism is supported between the inner and outer races. The engagement mechanism includes a first set of pawls and a second set of pawls that are operatively supported between the inner and outer races. In addition, the bi-direction over-running clutch assembly includes at least one actuating cam disposed adjacent the inner and outer races and including a plurality of disengagement portions as well as a plurality of engagement portions. The engagement and disengagement portions cooperate with the first and second pawls (1) to selectively disengage the first and second pawls to provide freewheeling relative rotation between the inner and outer races; (2) to selectively actuate at least one pawl of the first set of pawls so that torque is translated in a first rotational direction when the transmission assembly is in low gear but allowing freewheeling relative rotation between the inner and outer races in a second rotational direction that is opposite to the first rotational direction when the transmission assembly is in any higher gear; (3) to selectively actuate at least one pawl of the second set of pawls so that torque is translated in the second rotational direction opposite to the first rotational direction when the transmission assembly is in reverse gear but allowing freewheeling relative rotation in the first rotational direction when the transmission assembly is in any gear greater than first gear; and (4) to selectively actuate at least one pawl of the first and second sets of pawls so that torque is translated between the inner and outer races in both of the first and second rotational direction to provide engine braking.
Thus, one advantage of the bi-directional overrunning clutch assembly of the present invention is that it provides four distinct modes as illustrated in FIGS. 2-5. In this way, the bi-directional overrunning clutch assembly of the present invention may be employed as a component of an automotive driveline in a transmission, transfer case, or differential to eliminate other components while maintaining the requisite functionality.
Another advantage of the bi-directional overrunning clutch assembly of the present invention is that when it is used in connection with providing low and reverse gear ratios in the representative transmission illustrated in FIG. 6, at least one multi-disc friction clutch and a one-way clutch may be eliminated. In this way, the clutch assembly of the present invention reduces parasitic energy loss, improves operational efficiency, and reduces cost.
Another advantage of the bi-directional overrunning clutch of the present invention is that it may be employed in a transmission to provide the important engine braking effect that can occur when the speed of the transmission output shaft exceeds the speed of the input to any given planetary gear set.
Other objects, features and advantages of the present invention will be readily appreciated as the same becomes better understood after reading the subsequent description taken in connection with the accompanying drawings.